This invention relates generally to ultrasonic navigation systems, and more particularly, to a robotic control system which employs an array of rapid-firing ultrasonic transducers and fires same in accordance with a firing scheme which eliminates crosstalk interference between the ultrasonic transducers.
In order to detect obstacles and navigate around them, many mobile robots use multiple ultrasonic transducers or sensors. Typical configurations are comprised of 12, 16, 24, or even more sensors, which usually are installed along the periphery of the vehicle. It is well-known as a serious problem that ultrasonic sensors, particularly as they have heretofore been used in robotic navigation systems, are subject to noise and sporadic false readings. Ultrasonic sensors are additionally subject to the problems which plague any other sensor system.
An ultrasonic range sensor (URS) is comprised of a transducer with associated support electronics. The transducer used in the vast majority of all obstacle avoidance applications for mobile robots is manufactured by POLAROID CORPORATION, 119 Windsor Street, Cambridge, Mass. 02139, and was originally designed for automatic focusing of the POLAROID SX-70 camera. The transducer comprises of an ultrathin gold-plated membrane about 1.5" in diameter that can be used to either transmit or receive ultrasonic sound waves. With commercially available support electronics from TEXAS INSTRUMENTS, P.O. Box 225012, Dallas, Tex. 75265 (marketed by MICROMINT, INC, 4 Park Street, Vernon, Conn. 06066), the sensor has a maximum range of 10 m and a resolution of 3 cm. When triggered ("fired"), the known transducer system issues 16 pulses of ultrasound at 50 KHz, for the duration of 0.32 ms. The sound waves travel at a speed of 340 m/sec at room temperature. Objects located within a cone-shaped volume (the active cone) in front of the sensor reflect some of the incident sound energy back to the sensor. Shortly after firing, the ultrasonic sensor enters a receiving state and awaits the echo. When the echo reaches the sensor membrane, the resulting weak signal is amplified and thresholded. The time between firing the sensor and receiving the echo is called time-of-flight (TOF) and is directly proportional to the distance to the reflecting object. For example, if an object is 1 m ahead of the sensor, the corresponding TOF=1.times.2/340=5.9 ms.
The URS from TEXAS INSTRUMENTS is designed with an automatically increasing gain in the receiver circuit. This feature increases the gain approximately exponentially, as a function of the TOF. The variable gain is necessary to compensate for the exponential attenuation of the ultra-sound energy over distance. Note that the URS may interpret any ambient ultrasonic noise of sufficient amplitude as an echo, and thus produce an arbitrary, false range reading. Any firing algorithm can limit the time it is waiting for an echo. This measure reduces the maximum range of the sensor but increases the sampling rate. The time a URS is "open" to await an echo is termed the time window, T.sub.wind.
In most environments where a robotic system is required to operate, environmental ultrasonic noise is fairly rare. However, such is not the case in many manufacturing environments where hydraulic, pneumatic, and other systems which may be in use produce sufficient ultrasonic noise to render the sensors inoperable, even for short periods of time. Moreover, robots with multiple ultrasonic sensors may introduce their own noise, a phenomenon known as "crosstalk". Crosstalk differs from other environmental noise because it introduces a systematic error that will repeatedly cause similar erroneous reading. A related kind of "semi-systematic" error is crosstalk that may occur when multiple mobile robots with multiple ultrasonic sensors operate in the same environment. In most indoor applications, crosstalk is much more likely to occur than environmental ultrasonic noise.
When operating a mobile robot under such conditions, it is not practical to base the decision for an obstacle avoidance maneuver on a single (possibly erroneous) sensor reading that seems to indicate the presence of an object in front of the robot. There is a need for combining multiple range samples in order to increase the level of "confidence" which can be placed in the existence of an indicated obstacle, i.e., the signal-to-noise ratio. When traveling at relatively high speed, e.g., V&gt;0.3 m/sec, it is crucial to sample each sensor quickly and repeatedly so as to gather multiple samples within the time necessary to avoid a collision. It is a problem with known systems, however, that fast sampling of multiple sensors introduces an increased level of ultrasonic noise in the region of vehicle operation, and increases the occurrence rate of crosstalk.
The two foremost limitations of ultrasonic sensors are susceptibility to noise and susceptibility to specular reflections. With respect to noise, one can distinguish different sources of noise, namely environmental noise, noise from sensors on other mobile robots, and crosstalk from on-board ultrasonic sensors. Specular reflections occur when sound waves do not approach a surface frontally (.alpha.=0.degree.), but rather at a larger angle (e.g., .alpha.&gt;30.degree.). If the surface is smooth, it reflects the incident sound waves away from the sensor. Thus, the sensor does not receive an echo and consequently does not "see" the object.
For the POLAROID sensor and others known in the prior art, the active cone has an opening angle of approximately between 15.degree. and 30.degree.. A precise angle cannot be predetermined since the strength of the echo depends, in part, on characteristics of the surface and the orientation of the reflecting object. However, since 15.degree. is the more conservative assumption, many mobile robots have URSs installed on their periphery at 15.degree. intervals, to guarantee complete coverage of the area around the robot in all directions. For omnidirectional robots of circular shape, this design requires 24 (=360.degree./15.degree.) URSs mounted on a ring around the robot.
An even more important reason for using densely spaced, multiple URSs (arranged in at circular or semi-circular formation around the robot) lies in the ability of this arrangement to overcome, at least partially, the problem of specular reflections, where a mobile robot approaches a smooth wall. When the wall first comes into the range of one of the sensors, the angle .alpha. is too large and the incident beam is reflected away from the sensor. Consequently, the robot does not detect the object and continues to travel in the original direction, toward the obstacle. However, moments later the object appears in range of another of the sensors and is detected. A sensor ring with 15.degree. spacing between sensors will detect any object at any orientation, although the robot may have approached the object very closely before this happens. For this reason it is crucial to fire all sensors rapidly, so that an object is detected as soon as it comes in range of a suitable sensor.
Another reason for firing URSs rapidly is the detection of obstacles right in front of the robot. Clearly, there is a close relation between the firing rate of the sensors and the speed at which a mobile robot can travel safely. Since this relation depends strongly on the characteristics of objects in the environment, it is impossible to formulate this relation mathematically. A vague guideline, however, is the observed performance of the commercially available mobile robot from DENNING MOBILE ROBOTICS, INC., 21 Cummings Park, Woburn, Mass. 01801 that can safely avoid most obstacles at 0.3 m/sec, while each one of its 24 URSs is fired once in approximately 300 ms. This speed limit is not imposed by the motors or the on-board computing power (which could easily be improved). Since 0.3 m/sec is insufficient for most applications, there is a need to be able to increase the sampling rate of the ultrasonic sensors.
It is, therefore, an object of this invention to provide an ultrasonic control system which can operate in an environment of high ultrasonic noise generated by the ultrasonic transducers themselves.
It is another object of this invention to provide an ultrasonic navigation system which permits multiple robots to operate in the same environment.
It is also an object of this invention to provide an ultrasonic robotic navigation system which is reliable, robust, and which eliminates the effects of crosstalk.
It is a further object of this invention to provide a navigation system wherein mobile robots can safely traverse obstacle-cluttered environments several times faster than can be achieved with present systems.
It is additionally an object of this invention to provide a robotic navigation system wherein moving objects (e.g., people walking in the robot's path) can be avoided, even if they move at a higher speed than that of the robot.
It is yet a further object of this invention to provide a navigation system for nonvehicle applications, such as whole-arm protection for (stationary) robot arms or electronic travel aids for visually impaired people.
It is also another object of this invention to provide a navigation system which overcomes the problems associated with specular reflections.
It is yet an additional object of this invention to provide an ultrasonic navigation system for a robotic vehicle wherein high sampling rates are achieved to minimize the risk of collision of the vehicle, without unduly increasing the rate of occurrence of crosstalk.